1. Field of the Invention
The present invention relates to composite materials and in particular to prepregs which can be used in building composite materials of lay-ups of such prepregs which are pressmolded and cured.
2. Description of Related Art
The use of composite materials, in particular reinforced thermosetting composites, is continually increasing; a new application of such material is for windmill blades in wind energy plants.
Prepregs have been known and used for this purpose, which are composed of heat-curable resins and fibers and may also comprise a coarse net of reinforcing rovings. Such prepregs are sold to the customer who can form composite materials of different shapes by building up lay-ups of such prepregs, for instance 50 and more prepreg layers, and press molding and heating these lay-ups to obtain the appropriate shape and cure the resin.
In highly stressed components the void content of such laminates is significant for the performance and therefore for dimensioning such parts, as each void is a point of defect which decreases the mechanical properties. For this reason the customer requires prepregs which produce a low, reproducible void content, but which at the same time have good handling properties.
Since air tends to be captured between several layers of prepregs, it has been customary to process the lay-up of the prepregs under vacuum. It has also been known to intermittently interpose dry, air-permeable layers of for instance resin-free webs between the resin layers to allow the air to escape through these dry layers when putting on vacuum. This technique is rather troublesome and does not yield reproducible results, since in the heating step the resin penetrates the air-permeable layer irregularly. A technique of this kind is described in DE 202 01 902 U1, whereby the prepreg is combined with a web which over its thickness is only partially impregnated. It is difficult to fix such a thick web to the prepreg without applying a further resin layer at the top, possibly together with another fixing element, otherwise there will be loose filaments at the outer side of the prepreg, which would impair the handling properties.